Spark-ignition and diesel engine manufacturers are being challenged to produce engines that are both highly fuel efficient and meet increasingly stringent emission requirements. Diesel engines have high fuel efficiency but require expensive after-treatment to meet recently enacted very strict emissions regulations. Also, most diesel after-treatment systems require a urea/water solution and the tank must be refilled at regular intervals adding to the operating expense. In contrast, spark-ignition (SI) engine exhaust emissions can be cleaned up using relatively inexpensive after-treatment based on a three-way catalyst. However, SI engines suffer from relatively low fuel efficiency, typically consuming 30% to 50% more fuel than a diesel engine to do the same work. For these reasons, there is a strong need for an engine that has both high fuel efficiency (comparable to diesel efficiencies or higher) and that does not require expensive after-treatment.
Homogeneous charge compression ignition (HCCI) is an attractive advanced combustion process that offers potential as a high-efficiency alternative to spark ignition engines. By providing diesel-like efficiencies but with substantially lower nitrogen oxide (NOx) and particulate matter (PM) emissions, HCCI also offers a low emissions alternative to diesel engines, without expensive after-treatment. Unlike conventional diesel combustion, HCCI does not rely on maintaining a flame front. Rather, combustion occurs as the result of spontaneous auto-ignition at multiple points throughout the volume of the charge gas. This unique property of HCCI allows the combustion of very lean mixtures or mixtures that are made very dilute by the addition of combustion-product gases (e.g., by exhaust gas recirculation), resulting in low combustion temperatures that dramatically reduce NOx emissions. Also, unlike conventional diesel combustion, the charge is sufficiently well mixed so that PM emissions are very low. Consequently, HCCI provides a low emissions alternative to conventional diesel engines or a high efficiency alternative to conventional SI engines.
Although the use of conventional diesel fuel or gasoline for HCCI would be desirable since these fuels are readily available, achieving acceptable HCCI performance with these fuels can be difficult. With diesel fuel, elevated temperatures are required before significant vaporization occurs making it difficult to form a premixed near-homogeneous charge. Second, diesel fuel is a two-stage ignition fuel with significant low-temperature combustion chemistry, which leads to rapid auto-ignition once compression temperatures exceed about 800K. This can lead to overly advanced combustion phasing and/or require reduced compression ratios that reduce engine efficiency. Conversely, single-stage ignition fuels such as gasoline can require overly high compression ratios or various other techniques to provide significant charge heating, e.g., retaining significant amounts of hot combustion products from the previous combustion cycle (residuals) in the cylinder.
Currently, the power output of HCCI engines is limited to about half that of traditional diesel or SI engines. Extending HCCI operation to higher power outputs remains a significant challenge. This is mainly because the combustion rates with HCCI become very rapid as the fueling rate is increased causing engine knock that results in undesirable noise and reduced durability. Because high-load operation is a challenge, most HCCI or HCCI-like concepts currently being pursued utilize HCCI only below about half load and revert to conventional spark ignition (SI) or diesel combustion for high loads. Thus, at present the advantages afforded by HCCI are limited to only part of the operating range, and engine design must be compromised to accommodate the conventional diesel or SI combustion.
In an earlier work (U.S. Pat. No. 7,128,046, herein incorporated by reference) the inventors introduced a technique for reducing the combustion heat release rate (HRR) in HCCI engines, which gives a commensurate reduction in the knocking propensity. With this technique, termed partial fuel stratification (PFS), a large portion of the fuel (typically, more than half) is premixed and the remaining fuel is introduced by direct injection (DI) in the latter part of the compression stroke in such a manner so that it does not mix thoroughly, resulting in a partially stratified fuel/charge-gas mixture. Thus, the charge consists of a distribution of local fuel/charge-gas mixtures that auto-ignite at different times thereby producing a staged combustion event if the fuel's autoignition chemistry varies with the local fuel concentration within the charge gas.
If the charge consists only of air and fuel, then this variation in local fuel concentration is synonymous with variations in the local fuel/air equivalence ratio, φ, wherein, φ is defined by (F/A)/(F/A)stoichiometric, where F is the mass of fuel and A is the mass of air. However, in HCCI engines, it is often necessary to retain significant amounts of hot residuals (i.e., combustion-product gases) in the cylinder, and/or to use significant amounts of exhaust-gas recirculation (EGR). In these cases, the charge gas consists of fuel, air, and combustion-product gases, and variations in local fuel concentration can be cast in terms of a mass-based equivalence ratio φm, wherein φm is defined by (F/C)/(F/A)stoichiometric, wherein C is the mass of charge gases, excluding the fuel, and F and A are defined as above. It should be noted that φ and φm are identical when the charge consists fuel's autoignition chemistry must vary with the local φm, i.e., the fuel must be φm-sensitive. Thus, successful application of PFS requires both that fuel be introduced in a manner to produce an appropriate φm distribution and that the fuel be φm-sensitive, so that autoignition occurs sequentially in the various local φm regions. For most φm-sensitive fuels, this means that the richest regions (regions with the highest local φm) autoignite first, followed by the next richest, and so on. If this is done correctly, the HRR can be significantly reduced with a commensurate reduction in knocking propensity. However, not all fuels have the required φm-sensitivity. Two-stage-ignition fuels are generally φm-sensitive (in contrast to single-stage-ignition fuels such as gasoline) and the prior art method was specific to two-stage-ignition fuels. Diesel fuel is a commonly available two-stage-ignition fuel but it is difficult to use diesel fuel for HCCI due to its low volatility and overly rapid autoignition. Other two-stage-ignition fuels have sufficient volatility, but are not readily available. Consequently, the prior art method for improving combustion characteristics of HCCI engines is limited to fuels that are not readily available and/or would require extensive modification of HCCI engines to work properly.